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ECGD 4122 – Foundation Engineering Lecture 2 Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2 nd Semester 2008/2009.

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Presentation on theme: "ECGD 4122 – Foundation Engineering Lecture 2 Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2 nd Semester 2008/2009."— Presentation transcript:

1 ECGD 4122 – Foundation Engineering Lecture 2 Faculty of Applied Engineering and Urban Planning Civil Engineering Department 2 nd Semester 2008/2009

2 Revision of Soil Mechanics  Soil Composition  Soil Classification  Groundwater  Stress (Total vs. Effective)  Settlement  Strength 2

3 Soil: A 3-Phase Material Solid Water Air 3

4 The Mineral Skeleton Volume Solid Particles Voids (air or water) 4

5 Three Phase Diagram Solid Air Water Mineral Skeleton Idealization: Three Phase Diagram 5

6 Fully Saturated Soils Fully Saturated Water Solid Mineral Skeleton 6

7 Dry Soils Mineral Skeleton Dry Soil Air Solid 7

8 Partially Saturated Soils Solid Air Water Mineral SkeletonPartly Saturated Soils 8

9 Three Phase System Volume Weight Solid Air Water WTWT WsWs WwWw W a ~0 VsVs VaVa VwVw VvVv VTVT 9

10 Weight Relationships  Weight Components: Weight of Solids = W s Weight of Water = W w Weight of Air ~ 0 10

11 Volumetric Relationships  Volume Components: Volume of Solids = V s Volume of Water = V w Volume of Air = V a Volume of Voids = V a + V w = V v 11

12 Volumetric Relationships  Volume Components: Volume of Solids = V s Volume of Water = V w Volume of Air = V a Volume of Voids = V a + V w = V v 12

13 Specific Gravity  Unit weight of Water,  w  w = 1.0 g/cm 3 (strictly accurate at 4° C)  w = 62.4 pcf  w = 9.81 kN/m 3 13

14 Specific Gravity, G s  Iron7.86  Aluminum  Lead11.34  Mercury13.55  Granite2.69  Marble2.69  Quartz2.60  Feldspar

15 Specific Gravity, G s 15

16 Example: Volumetric Ratios  Determine void ratio, porosity and degree of saturation of a soil core sample Data:  Weight of soil sample = 1013g  Vol. of soil sample = 585.0cm 3  Specific Gravity, G s = 2.65  Dry weight of soil = 904.0g 16

17 Solid Air Water W a ~0 Volumes Weights g 585.0cm g  s = g 341.1cm cm cm cm 3  W =1.00 Example 17

18 585.0cm 3 Solid Air Water Volumes  s = cm cm cm cm 3  W =1.00 Example 18

19 Soil Unit weight (lb/ft 3 or kN/m 3 )  Bulk (or Total) Unit weight  = W T / V T  Dry unit weight  d = W s / V T  Buoyant (submerged) unit weight  b = -  w 19

20 Typical Unit weights 20

21 Fine-Grained vs. Coarse-Grained Soils  U.S. Standard Sieve - No inches mm  “ No. 200 ” means... 21

22 Sieve Analysis ( Mechanical Analysis)  This procedure is suitable for coarse grained soils  e.g. No.10 sieve …. has 10 apertures per linear inch 22

23 Hydrometer Analysis  Also called Sedimentation Analysis  Stoke’s Law 23

24 Grain Size Distribution Curves 24

25 Soil Plasticity  Further classification within fine-grained soils (i.e. soil that passes #200 sieve) is done based on soil plasticity.  Albert Atterberg, Swedish Soil Scientist ( ) …..series of tests for evaluating soil plasticity  Arthur Casagrande adopted these tests for geotechnical engineering purposes 25

26  Consistency of fine-grained soil varies in proportion to the water content Atterberg Limits Shrinkage limit Plastic limit Liquid limit solid semi-solid plastic liquid Plasticity Index (cheese) (pea soup) (pea nut butter) (hard candy) 26

27 Liquid Limit (LL or w L )  Empirical Definition  The moisture content at which a 2 mm- wide groove in a soil pat will close for a distance of 0.5 in when dropped 25 times in a standard brass cup falling 1 cm each time at a rate of 2 drops/sec in a standard liquid limit device 27

28 Engineering Characterization of Soils Soil Properties that Control its Engineering Behavior Particle Size Particle/Grain Size Distribution Particle Shape Soil Plasticity fine-grainedcoarse-grained 28

29 Clay Morphology  Scanning Electron Microscope (SEM)  Shows that clay particles consist of stacks of plate-like layers 29

30 Soil Consistency Limits  Albert Atterberg ( ) Swedish Soil Scientist ….. Developed series of tests for evaluating consistency limits of soil (1911)  Arthur Casagrande ( ) …… A dopted these tests for geotechnical engineering purposes 30

31 Arthur Casagrande ( )  Joined Karl Terzaghi at MIT in 1926 as his graduate student  Research project funded by Bureau of Public Roads  After completion of Ph.D at MIT Casagrande initiated Geotechnical Engineering Program at Harvard  Soil Plasticity and Soil Classification (1932) 31

32 Casagrande Apparatus 32

33 Casagrande Apparatus 33

34 Casagrande Apparatus 34

35 Liquid Limit Determination 35

36  The moisture content at which a thread of soil just begins to crack and crumble when rolled to a diameter of 1/8 inches Plastic Limit (PL, w P ) 36

37 Plastic Limit (PL, w P ) 37

38 Plasticity Index ( PI, I P )  PI = LL – PL or I P =w L -w P  Note: These are water contents, but the percentage sign is not typically shown. 38

39 Plasticity Chart 39

40 USCS Classification Chart 40

41 USCS Classification Chart 41

42 Plasticity Chart 42

43 Groundwater U = porewater pressure =  w Z w 43

44 Stresses in Soil Masses Area = A  = P/A X X Soil Unit P Assume the soil is fully saturated, all voids are filled with water. 44

45 Effective Stress  From the standpoint of the soil skeleton, the water carries some of the load. This has the effect of lowering the stress level for the soil.  Therefore, we may define effective stress = total stress minus pore pressure ′ =  - u where, ′ = effective stress  = total stress u = pore pressure 45

46 Effective Stress ′ =  - u  The effective stress is the force carried by the soil skeleton divided by the total area of the surface.  The effective stress controls certain aspects of soil behavior, notably, compression & strength. 46

47 Effective Stress Calculations ′ z =   i H i - u where, H = layer thickness  sat = saturated unit weight U = pore pressure =  w Z w When you encounter a groundwater table, you must use effective stress principles; i.e., subtract the pore pressure from the total stress. 47

48 Geostatic Stresses 48

49 Compressibility & Settlement  Settlement requirements often control the design of foundations  This chapter provides a general overview of principles involved in settlement analysis  The subject will be dealt with in greater detail in Chapter 7. 49

50 Increase in Vertical Effective Stress Due to a Placement of a fill Due to an external load 50

51 Voids Solids H V v = eV s VsVs  c c  e e V v = (e -  e)V s VsVs Solids z′z′ z′z′  z0 ′ }  z f ′ Before After Consolidation 51

52  u0u0 00 Before Loading  Point, P 52

53  u0+uu0+u  0 +  Immediately After Loading  Point, P 53

54  0 +   u0+uu0+u  u0u0 Shortly after Loading No settlement Long after Loading Settlement Complete 54

55 Settlement Distortion Settlement (Immediate) Consolidation (Time Dependent) Secondary Compression Time Settlement 55

56 Laboratory Consolidation Test 56

57 Consolidation Test 57

58 Test Results 58

59 Consolidation Plot 59

60 Test Results Idealized Data 60

61 Compression Index and Recompression Index 61

62 Compression Ratio and Recompression Ratio 62

63 Normally and Over-Consolidated Soils ….. Normally consolidated ….. Over consolidated ….. Under consolidated 63

64 Over-Consolidation Margin & Over- consolidation Ratio ….. Over-consolidation Margin ….. Over consolidation ratio 64

65 Typical Range of OC Margins 65

66 Compressibility of Sand and Gravels (Table 3.7) 66

67 Example 3 67

68 Settlement Predictions N.C. Clays 68

69 Settlement Predictions O.C. Clays…… Case I 69

70 Settlement Predictions O.C. Clays…… Case II 70

71 Example 4 71

72 Example 4 72

73 Example 5 73

74 Example 5 74

75 Slope Failure in Soils Failure due to inadequate strength at shear interface 75

76 Shear Failure in Soils 76

77 Shear Failure in Soils 77

78 Bearing Capacity Failure 78

79 Transcosna Grain Elevator Canada (Oct. 18, 1913) West side of foundation sank 24-ft 79

80 Shear Strength of Soils  Soil derives its shear strength from two sources: Cohesion between particles (stress independent component)  Cementation between sand grains  Electrostatic attraction between clay particles Frictional resistance between particles (stress dependent component) 80

81 Shear Strength of Soils; Cohesion Dry sand with no cementation Dry sand with some cementation Soft clay Stiff clay 81

82 Shear Strength of Soils; Internal Friction 82

83 Shear Strength, S Normal Stress,  =  C  =  Mohr-Coulomb Failure Criterion 83

84 Shear Strength is controlled by Effective Stress,  ' Potential Failure Surface Slope Surface 84

85 Mohr-Coulomb Failure Criterion 85

86 Typical  Values 86

87 Effect of Pore Water on Shear Strength  Pore water pressure  Total Stress, versus Effective Stress,  Shear Strength in terms of effective stress 87

88  Moist beach sand has apparent cohesion  Negative pore water pressures Apparent Cohesion 88

89 Measuring Shear Strength Laboratory  Direct shear test  Unconfined compression test  Triaxial compression test Field  Vane shear test 89

90 Direct Shear Test ASTM D-3080; AASHTO T

91 Direct Shear Test 91

92 Direct Shear Test 92

93 Direct Shear Test Device 93

94 Direct Shear Test Device 94

95 Direct Shear Test Data Shear stress 95

96 Direct Shear Test Data Volume change 96

97 Peak vs. Ultimate Strength 97

98 Example: Direct Shear Test Given: A direct shear test conducted on a soil sample yielded the following results: Normal Stress,  (psi) Max. Shear Stress, S (psi) Required: Determine shear strength parameters of the soil 98

99 Example 6 99

100 Drained versus Undrained Conditions …. Before loading After loading 100

101 Drained versus Undrained Conditions …. Before loading After loading 101

102 102

103 103

104  Drained conditions occur when rate at which loads are applied are slow compared to rates at which soil material can drain  Sands drain fast; therefore under most loading conditions drained conditions exist in sands  Exceptions: pile driving, earthquake loading in fine sands Soil Shear Strength under Drained and Undrained Conditions …. 104

105  In clays, drainage does not occur quickly; therefore excess pore water pressure does not dissipate quickly  Therefore, in clays the short-term shear strength may correspond to undrained conditions  Even in clays, long-term shear strength is estimated assuming drained conditions Soil Shear Strength under Drained and Undrained Conditions …. 105

106 Shear Strength in terms of Total Stress  Shear Strength in terms of effective stress Shear strength in terms of total stress u at hydrostatic value 106

107 Long-term Stability Potential Failure Surface Slope Surface 107

108 Short-term Stability Potential Failure Surface Slope Surface 108

109 Shear Strength in terms of Total Stress;  = 0 condition  Shear strength in terms of total stress  For cohesive soils under saturated conditions,  =

110 Shear Strength, S Normal Stress,  C  = 0 Mohr-Coulomb Failure Criterion 110

111 Mohr’s Circles  3 =0 11 Direct Shear Uniaxial Compression 111

112 Mohr’s Circles  3 =0 11 Uniaxial Compression   11 Horiz. plane Max. shear plane 112

113 Mohr’s Circles  3 =0 11 Uniaxial Compression 113

114 Unconfined Compression Test ASTM D-2166; AASHTO 208  For clay soils  Cylindrical specimen  No confining stress (i.e.  3 = 0)  Axial stress =  1  3 = 0 11 114

115 Unconfined Compression Test Data 115

116 Unconfined Compression Test 116

117 Example: Unconfined Compression Test Given: An unconfined compression test conducted on a soil sample yielded the results shown in the table. Required: Determine undrained shear strength, S u of the soil 117

118 Example: Unconfined Compression Test 118

119 Example: Unconfined Compression Test q u = 43.45psi=6257 psf S u = 21.7psi = 3128 psf 119

120 Triaxial Compression Test  Unconfined compression test is used when  = 0 assumption is valid  Triaxial compression is a more generalized version  Sample is first compressed isotropically and then sheared by axial loading 11 33 120

121 Triaxial Compression Test  Load applied in 2 stages confining pressure,  3 dev. stress,  =  1 -  3 11 33 33 121

122 Triaxial Compression Test 122

123 Triaxial Compression Test 123

124 Triaxial Compression Test for Undisturbed Soils   124

125 Drainage during Triaxial Compression Test 125

126 Triaxial Compression Tests  Unconsolidated Undrained (UU- Test); Also called “ Undrained ” Test  Consolidated Undrained Test (CU- Test)  Consolidated Drained (CD-Test); Also called “ Drained Test ” 126

127 Triaxial Compression Tests ASTM Standards  ASTM D2850: Unconsolidated Undrained Triaxial Test for Cohesive Soils  ASTM D4767: Consolidated Undrained Triaxial Test for Cohesive Soils 127

128 Triaxial Compression Tests AASHTO Standards  AASHTO T-296: Unconsolidated Undrained Triaxial Test for Cohesive Soils  AASHTO T-297 : Consolidated Undrained Triaxial Test for Cohesive Soils 128

129 Consolidated Undrained Triaxial Test for Undisturbed Soils 129


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